It appears to me that this question was already answered on
https://q2a.cs.uni-kl.de/552/exercise-sheet-2-question-2-c. Have a look at it.
At the end of the computation, a node should be created with label b and children N_1 and N_2, i.e, (b ? N_1 : N_2). However, before creating new nodes, BDD packages consult their unique table to check whether such a node already exists. In our case, such a node will be found under the address N_22. So, no new node for (b ? N_1 : N_2) is created, and instead, simply N_22 is returned.
